This invention relates to a porous body for an electrolytic capacitor into which lead wires are implanted, and to a method of producing the same. More particularly, the present invention relates to an aluminum-titanium alloy electrolytic capacitor and to a method of producing the same.
So-called valve metals, such as tantalum, niobium, zirconium, vanadium, hafnium, titanium and aluminum are known as anode materials for an electrolytic capacitor. Conventionally, intensive studies have been made to examine the fundamental characteristics of these metals for use as an electrolytic capacitor in the form of either a single substance or an alloy and to put them into practical application.
In order to put these metals into a practical application as the capacitor, however, the leakage current, dielectric loss, and the like, of the oxide films must reach predetermined levels. For this reason, only tantalum and aluminum have been put into practical applications at present as the anode metals of the capacitor.
The capacitor using tantalum as the anode material has the advantages that its electric characteristics such as the leakage current and dielectric loss are excellent, it is electrically stable, has exttremely high reliability, is small in size, but it still provides a high capacity. However, supply cannot follow the demand of tantalum in recent years due to its limited resources, and the price is soaring, thereby raising the production cost of the capacitor.
The capacitor using aluminum as the anode material is less expensive, but it involves the poblem that both miniaturization and increase of capacity are difficult to attain simultaneously. Moreover, the electric characteristics and stability of the capacitor using aluminum are inferior to those of the capacitor using tantalum as the anode material.
Under the circumstances described above, the art has long sought to develop an electrolytic capacitor using, as the anode, those materials which have excellent electric characteristics such as low leakage current and dielectric loss, and excellent reliability, and which can be supplied stably and economically. As a result of intensive studies, the inventors found that these requirements can be satisfied by a porous sintered body for an electrolytic capacitor using an aluminum-titanium alloy as the anode material. Such porous Al-Ti alloy is disclosed in U.S. Pat. No. 4,331,477 which was issued and assigned to the assignee of the present invention on May 25, 1982.
A porous Al-Ti alloy body can be obtained by the following steps, for example. First, mix aluminum and either titanium or titanium hydride (TiH2) in the form of powder in the micron order. The mixed powder is then press-molded with a lead wire embedded in it. The mold is sintered at a vacuum of 1.times.10.sup.-6 mmHg at a temperature of 1,000.degree. to 1,100.degree. C. for 1 to 3 hours, to obtain an alloy porous sintered body. The (Al-Ti) alloy reaction occurs during sintering and aluminum is exclusively diffused into and absorbed by titanium, so that the portions where aluminum has been previously present become and remain porous, thereby providing the alloy porous body.
The Al-Ti alloy having the excellent capacitor characteristics also has the characterizing feature that a porous body having a large specific surface area can be obtained easily. This characterizing feature results from the fact that the alloying reaction between Al and Ti has a high unidirectionality of diffusion of Al into Ti.
The reaction sintering process of the Al-Ti or Al-TiH.sub.2 mixed powder utilizes the peculiarity of this alloying reaction. The skeleton structure and specific surface area of the porous body in this case are fundamentally determined by the particle size distribution of the Ti or TiH.sub.2 powder and the porous structure, and by the particle size distribution of the Al powder.
In such an Al-Ti porous body, the specific surface area of approximately 1 m.sup.2 /cm.sup.3, which corresponds to CV/volume of 75,000 .mu.F.multidot.V/cm.sup.3, can be obtained easily. Hence, this porous body is extremely advantageous for obtaining an electrolytic capacitor which is small in size, but which has a large capacitance.
This novel solid electrolytic capacitor is indeed revolutionary in that it has the characterizing features of a tantalum electrolytic capacitor (such as a small size) and a large capacitance, and the characteristic features of an aluminum electrolytic capacitor (such as low cost).
The problem of incorporating a lead wire is serious for the Al-Ti capacitor, because the Al-Ti alloy which is approximate to the composition of the porous body is extremely brittle, which is the characterizing feature inherent to intermetallic compounds. Thus, it is extremely difficult to prepare a wire from the alloy. For this reason, the same material that is used to make the porous body cannot also be used to make the lead wire. This is unlike tantalum, which may be used to make both a capacitor and the lead wire. It is, therefore, of the utmost importance to find other materials of the lead wire that can substitute for the Al-Ti alloy.
However, the material of the lead wire for the Al-Ti capacitor must have the anodization characteristics which are at least compatible with the anodization of the Al-Ti alloy, and the material must be sufficiently economical. In order to use the material as the embedded lead wire, the material must be resistant to the baking and sintering temperatures and must also have good sinterability with the porous body.
In the light of these requirements, Al and Ta that have been used as the material for the electrolytic capacitor are not suitable, because the melting point of Al is too low and Ta is too expensive. Having a high melting point, Ta has low sinterability. Other valve metals are mostly expensive and their formation characteristics are not very satisfactory.
Accordingly, the inventors of the present invention have realized a lead wire, which is substantially homogeneous to the porous body, by alloying the surface of titanium, which is one of the constituent elements of the Al-Ti alloy and is relatively economical, with aluminum.
Various methods may be used to alloy the surface of the Ti wire but the most simple method is to alloy the Ti surface by the Al vapor generated from the porous body, during sintering.
Accordingly, if suitable conditions are selected, it is expected that the surface of the Ti wire can be sufficiently alloyed during heat-treatment.
However, the following problems were found when the Ti wire was used.
First, when the Al-Ti mixed powder composition of the press-molded article is not Al-rich, that is, when the Al content is below about 51 atom % and/or when the sintering temperature is low, the Al vapor becomes insufficient and the wire surface composition of the portion embedded into the sintered body becomes Al-poor, so that the Ti wire surface is not sufficiently alloyed. Under such a state, the formation characteristics, such as a leakage current (LC), deteriorate severely.
Second, a bending deformation of the Ti lead wire is liable to occur at the wire portion outside the sintered body, after sintering. This problem of wire bending is serious when the material is put on an automatic mass-production line. By all means, this problem must be solved.